In an electrographic developing apparatus of this type, the toner contained in the developer in a developer container is consumed by the development of latent electrostatic images, so that there is a need to replenish the developer in the container with toner to compensate for the gradual decrease of the amount of toner and keep the proportion of toner, i.e. the toner concentration, uniform at all times.
Accordingly various toner supply devices have been proposed. Such devices include one which is adapted to directly detect the toner concentration of the developer accommodated in the developer container and replenish the developer with a suitable amount of toner in accordance with the detected value. Another proposed device utilizes the fact that the decrease in the overall amount of developer in the developer container is approximately in proportion to the reduction in the toner concentration of the developer, and it is adapted to detect the amount of developer in the container and to replenish the developer with a suitable amount of toner in accordance with the detected value.
Such toner supply devices, when used in an electrographic developing apparatus, make it possible to automatically replenish the developer in the developer container with toner, but the devices require a complex detecting circuit or the like and further involve many problems in reliably maintaining the toner concentration of the developer at a constant level at all times. Thus the devices should be improved still further before they can be put to practical use in an electrographic developing apparatus.